Lithium secondary battery manufacturing method

Abstract

A method for manufacturing a lithium secondary battery according to the present invention includes (a) a battery cell assembling step of housing an electrode assembly in a battery case and injecting an electrolyte, and (b) a pre-aging step of maturing the battery cell into which the electrolyte has been injected. The pre-aging step (b) includes (b-1) a sound wave application step of applying sound waves to the assembled battery cell using a speaker, and (b-2) an aging step of maturing the battery cell. The method for manufacturing a lithium secondary battery according to the present invention includes a process for removing air bubbles from inside the battery using a speaker, which has the effect of improving the impregnation of the electrolyte and shortening the time required for the pre-aging step.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0188409, filed on December 27, 2021.

[0002] The present invention relates to a method for manufacturing a lithium secondary battery, and more particularly, to a method for manufacturing a lithium secondary battery, which includes a step of removing air bubbles using a speaker immediately after injecting an electrolyte, thereby improving impregnation of the electrolyte. [Background technology]

[0003] In recent years, with the technological development and increasing demand for mobile devices, the demand for secondary batteries as energy sources has been rapidly increasing. Among these secondary batteries, much research has been conducted on lithium secondary batteries, which have high energy density and discharge voltage, and they have been commercialized and widely used.

[0004] A lithium secondary battery has a structure in which a porous separator is interposed between an electrode assembly, in which an active material is coated on a positive electrode and an active material is coated on a negative electrode, and a non-aqueous electrolyte containing a lithium salt is impregnated in the electrode assembly.

[0005] A typical lithium secondary battery is assembled by fabricating an electrode assembly consisting of alternately stacked positive and negative electrodes with a separator between them. The electrode assembly is then inserted into a battery case, typically a can or pouch, of a certain size and design, and finally an electrolyte is injected. The electrolyte seeps into the spaces between the positive and negative electrodes and the separator due to capillary force. However, due to the characteristics of the materials, the positive and negative electrodes and the separator are all highly hydrophobic, while the electrolyte is hydrophilic. Therefore, it takes a considerable amount of time and strict process conditions to increase the electrolyte's wetting of the electrodes and separator.

[0006] In particular, in the case of a jelly-roll type electrode assembly, as the internal structure of the battery case becomes more complex in order to increase the capacity and resistance of the battery, it takes a long time for the electrolyte to penetrate into the jelly-roll type electrode assembly. If air bubbles are generated inside, the electrolyte does not easily penetrate into the jelly-roll type electrode assembly even after a sufficient amount of time has passed since the electrolyte was injected. Therefore, a process for removing such air bubbles is required.

[0007] Meanwhile, Korean Patent Publication No. 10-2015-0162916 discloses an electrolyte impregnation device that includes a holder for fixing a battery cell and a resonant vibration application unit that applies ultrasonic waves to the holder to improve the impregnation of the electrolyte, and also attempts to use such ultrasonic waves to remove air bubbles. However, a liquid medium is required for efficient transmission of ultrasonic waves, significantly increasing the complexity of the process. In addition, energy is also transmitted to the electrodes inside the battery during the ultrasonic application process, which can cause side effects such as the detachment of some of the active material from the coated active material layer.

[0008] Therefore, there is a strong need for a technology that can fundamentally solve these problems. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to solve the above-mentioned problems of the prior art and the technical problems that have been required in the past.

[0010] Specifically, an object of the present invention is to provide a method for manufacturing a lithium secondary battery that removes air bubbles present inside a battery case to improve the impregnation of the electrolyte.

[0011] Another object of the present invention is to improve the impregnation of the electrolyte by removing bubbles generated inside the battery, thereby shortening the waiting time for impregnation of the electrolyte and uniformly impregnating the electrolyte, thereby improving the charging uniformity of the electrodes during the subsequent charging process. [Means for solving the problem]

[0012] A method for manufacturing a lithium secondary battery according to one embodiment of the present invention includes (a) a battery cell assembling step of housing an electrode assembly in a battery case and injecting an electrolyte solution, and (b) a pre-aging step of aging the battery cell into which the electrolyte solution has been injected. The (b) pre-aging step includes (b-1) a sound wave applying step of applying sound waves to the assembled battery cell using a speaker, and (b-2) an aging step of aging the battery cell.

[0013] In one embodiment of the present invention, the speaker is a directional speaker.

[0014] In one embodiment of the present invention, in the step (b-1) of applying sound waves, sound waves are applied locally to an area where bubbles are expected to be generated.

[0015] In an embodiment of the present invention, in the step (b-1) of applying sound waves, the sound waves are applied by sweeping the frequency of the sound waves.

[0016] The method for manufacturing a lithium secondary battery according to one embodiment of the present invention further includes, after the pre-aging step (b), an activation charging step (c) of charging the battery cell.

[0017] In an embodiment of the present invention, the step (b-1) of applying sound waves may involve applying sound waves to the battery cell in a sealed state.

[0018] In one embodiment of the present invention, the step (b-1) of applying sound waves may be performed without sealing the battery cell.

[0019] In one embodiment of the present invention, in the step (b-1) of applying sound waves, the frequency of the sound waves from the directional speaker is 20 Hz to 20,000 Hz.

[0020] In one embodiment of the present invention, in the step (b-1) of applying sound waves, the carrier wave of the directional speaker is an ultrasonic wave.

[0021] In one embodiment of the present invention, in the step (b-1) of applying sound waves, the frequency of the carrier wave of the directional speaker is 20 kHz to 100 kHz. [Effects of the Invention]

[0022] The method for manufacturing a lithium secondary battery according to the present invention includes a process of applying sound waves intensively to a region where bubbles are expected to be generated during a pre-aging process for impregnation of an electrolyte solution, thereby successfully removing bubbles and improving the impregnation of the electrolyte solution.

[0023] In addition, the removal of air bubbles improves the impregnation of the electrolyte, thereby shortening the time required for the pre-aging step for impregnating the electrolyte. Furthermore, the removal of air bubbles allows the electrolyte to be uniformly impregnated into the electrodes, improving the charging uniformity of the electrodes during the subsequent charging process. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a flowchart of a method for manufacturing a lithium secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Because the present invention can be modified in various ways and can take various forms, specific embodiments are illustrated in the drawings and described in detail herein, but it is not intended to limit the invention to the particular forms disclosed, and it should be understood that the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

[0026] In this application, the terms "comprise" and "have" are intended to specify the presence of any feature, number, step, operation, component, part, or combination thereof stated in the specification, but should be understood as not precluding the presence or possible addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0027] 1 is a flowchart showing a method for manufacturing a lithium secondary battery according to one embodiment of the present invention. Referring to FIG. 1, the method for manufacturing a lithium secondary battery according to one embodiment of the present invention includes (a) a battery cell assembling step of placing an electrode assembly in a battery case and injecting an electrolyte solution, and (b) a pre-aging step of aging the battery cell after the electrolyte solution has been injected. The pre-aging step (b) includes (b-1) a sound wave applying step of applying sound waves to the assembled battery cell through a speaker, and (b-2) an aging step of aging the battery cell.

[0028] The method for manufacturing a lithium secondary battery according to the present invention is characterized in that after injecting an electrolyte, sound waves are applied locally to a portion of the battery cell where bubbles are expected to form, using a speaker, to remove bubbles present inside the assembled battery cell. When the sound waves are applied, the vibration energy of the sound waves removes bubbles in the electrode assembly or the electrolyte, thereby improving the impregnation of the electrolyte.

[0029] The method for manufacturing a lithium secondary battery of the present invention may be applied to any of cylindrical lithium secondary batteries, prismatic lithium secondary batteries, and pouch-type lithium secondary batteries, but is particularly useful for prismatic lithium secondary batteries or cylindrical lithium secondary batteries including a jelly-roll type electrode assembly, in which non-uniform impregnation is likely to occur between the center and outer portions of the winding.

[0030] A jelly roll-type electrode assembly according to one embodiment of the present invention has a wound shape in which electrodes and separators are alternately stacked. The electrodes may be a first electrode and a second electrode having a polarity opposite to that of the first electrode. The first electrode may be a negative electrode, and the second electrode may be a positive electrode. The separator may be interposed between the first electrode and the second electrode.

[0031] The above-mentioned step (a) of assembling a battery cell is a step of placing an electrode assembly in a battery case and injecting an electrolyte. Battery cases that house the electrode assembly can be classified into square, coin, cylindrical, pouch, etc. depending on their shape. The method of housing the electrode assembly in the case varies slightly depending on the shape (the specific details are obvious to those skilled in the art and will not be described in detail here). Next, a process of injecting an electrolyte into the battery case is performed.

[0032] After the electrolyte injection is completed, a (b) pre-aging step is performed to age the battery cell so that the electrolyte is impregnated into the electrode assembly. The pre-aging step of the present invention includes a (b-1) sound wave application step in which sound waves are applied to the battery cell using a speaker to remove air bubbles.

[0033] The (b-1) sonic wave application step of the present invention may involve applying sonic waves locally to an area where bubbles are expected to form. When sonic waves are applied, the vibration energy of the sonic waves has the effect of removing bubbles, but this vibration energy of the sonic waves may also act on the electrode active material layer, causing some of the electrode active material to detach. Therefore, applying sonic waves locally to an area where bubbles are expected to form can reduce the negative impact of the sonic waves on the electrode active material layer.

[0034] In this way, in order to locally apply sound waves to the area where bubbles are expected to be generated, it is preferable to select a speaker as a means for applying sound waves, and to select a directional speaker.

[0035] A directional speaker is a speaker with strong directivity that radiates sound waves in a desired direction. While a typical speaker emits sound waves that spread in all directions, a directional speaker does not spread sound waves, but transmits sound waves in only one direction, allowing sound waves to be transmitted only in the desired direction.

[0036] A directional speaker according to one embodiment of the present invention is a device that reproduces an audio signal in the audible band from ultrasonic waves above the audible band, and is an advanced acoustic element that synthesizes ultrasonic waves and sound waves. The directional speaker of the present invention carries an audio signal (sound wave) in the audible band on an ultrasonic band signal and radiates it into the air, and the air regenerates the audio signal within the directional angle of the ultrasonic wave, radiating sound only in a specific area. Here, the ultrasonic band signal is used as a carrier wave.

[0037] The directional speaker can apply sound waves intensively to areas where bubbles are expected to form, and is therefore highly effective in removing bubbles.

[0038] Because bubbles are likely to exist in the electrolyte or at the edge of the electrode assembly, the location to which sound waves are applied using a speaker may be one or more selected from the top surface of the battery, the bottom surface of the battery, and the side surface of the battery. Specifically, in the case of a cylindrical or prismatic battery, sound waves may be applied to the top or bottom surface of the battery, and in the case of a pouch-type battery, sound waves may be applied to the edge portion of the body of the battery, i.e., the "H"-shaped portion. In addition, in the case of a jelly-roll type electrode assembly, sound waves may be applied to the center of the electrode assembly because the center is more likely to be impregnated with electrolyte than the outer periphery.

[0039] In one specific example, in the (b-1) sound wave applying step, the frequency of the sound waves from the directional speaker may be 20 Hz to 20,000 Hz, preferably 30 Hz to 15,000 Hz, and more preferably 50 Hz to 10,000 Hz.

[0040] In the (b-1) sound wave applying step, the carrier wave of the directional speaker is an ultrasonic wave, and the frequency of the carrier wave may be 20 kHz to 100 kHz, preferably 20 kHz to 60 kHz.

[0041] In the above (b-1) sound wave applying step, the sound waves from the directional speaker may have a constant frequency or may be swept over a predetermined frequency range. Every object has one or more natural frequencies that vibrate at a specific frequency. When vibrations having the same frequency (resonant frequency) as a specific natural frequency of an object are applied from the outside, the amplitude of the object increases. Therefore, when the sound waves are applied by sweeping the frequency of the sound waves, an excellent effect of removing bubbles can be achieved.

[0042] Air bubbles inside a battery may exist in the electrolyte or inside the electrode assembly. In the present invention, however, in the (b-1) sound wave application step, sound waves of various frequencies are applied by sweeping the frequency, so that the electrolyte or electrode material resonates, thereby eliminating the air bubbles regardless of their size.

[0043] In applying sound waves in the sound wave application step (b-1), the time for applying sound waves once may be 10 seconds to 30 minutes, preferably 1 minute to 15 minutes. The total number of times sound waves are applied may be, specifically, 1 to 10 times, 2 to 9 times, or 3 to 8 times. The time interval between the application of sound waves and the next application may be 10 seconds to 10 minutes, preferably 15 seconds to 7 minutes, and more preferably 30 seconds to 5 minutes. The total time for applying sound waves may be 2 minutes to 60 minutes, 3 minutes to 45 minutes, or 5 minutes to 30 minutes.

[0044] The step (b-1) of applying sound waves may be performed by applying sound waves to the battery cell in a sealed state, or by applying sound waves to the battery cell in an unsealed state.

[0045] The (b-2) aging step of aging the battery cell is a step of leaving the battery cell under constant temperature conditions for a predetermined time so that the electrolyte is impregnated into the electrode assembly.

[0046] In one embodiment, the (b-2) aging step includes aging the battery cell at a normal temperature of 18°C to 28°C or at a high temperature of 40°C to 70°C.

[0047] When the pre-aging step is performed at room temperature, it may be performed under room temperature conditions of 18°C to 28°C, specifically 19°C to 27°C, more specifically 20°C to 26°C, and even more specifically 21°C to 25°C, but is not necessarily limited thereto and may be suitably changed depending on the characteristics of the battery to be designed.

[0048] When the pre-aging step is performed at a high temperature, it may be performed under high temperature conditions of 40°C to 70°C. Specifically, it may be performed at 45°C to 65°C, more specifically 44°C to 56°C, and even more specifically 50°C to 60°C, but it is not necessarily limited thereto and may be suitably changed depending on the characteristics of the battery to be designed. When pre-aging is performed at a high temperature, there is an advantage that the impregnation speed of the electrolyte can be further increased.

[0049] The time required for the aging step (b-2) may be specifically 3 to 60 hours, 6 to 48 hours, or 12 to 36 hours, and this may be suitably adjusted depending on the materials of the positive electrode, negative electrode, and electrolyte, the design capacity of the secondary battery, etc.

[0050] The method for manufacturing a lithium secondary battery of the present invention includes, after the (b) pre-aging step, (c) an activation charging step of charging and activating the battery cell. The (c) activation charging step is a step of charging the secondary battery to form an SEI (Solid electrolyte interface, hereinafter referred to as "SEI") coating layer on the negative electrode, and is a process of charging the assembled secondary battery to a SOC level within a predetermined range of full charge capacity (SOC100, State of Charge). Here, the predetermined SOC range may be 20% to 80%, more preferably 40% to 70%.

[0051] During the primary charge stage of a lithium secondary battery, lithium ions from the positive electrode active material migrate to the negative electrode. These highly reactive lithium ions react with the negative electrode to form compounds such as Li2CO3, LiO, and LiOH, forming an SEI film on the surface of the negative electrode. The SEI film is a non-conductor that forms when the amount of ion movement in the battery increases. The formation of the SEI film prevents the lithium ions from reacting with other materials at the negative electrode during subsequent charging of the secondary battery, and acts as a kind of ion tunnel, allowing only the lithium ions to pass through. The formation of this SEI film prevents lithium ions from reacting with the negative electrode or other materials, thereby reversibly maintaining the amount of lithium ions and enabling the secondary battery to be charged and discharged reversibly, thereby extending the life of the secondary battery. Furthermore, because it is not easily degraded even when left at high temperatures or subjected to repeated charging and discharging, it also reduces changes in battery thickness.

[0052] The charging conditions for the primary charging stage may be those known in the art. Specifically, the charging method may be a constant current charging method until the end-of-charge voltage is reached. The charging rate (c-rate) may be 0.01 C to 2 C, 0.1 C to 1.5 C, or 0.2 C to 1 C, but is not limited thereto and may be suitably changed depending on the characteristics of the positive and negative electrode materials.

[0053] The primary charging step may be carried out at a temperature condition of 18°C to 28°C, specifically 19°C to 27°C, and more specifically 20°C to 26°C.

[0054] The primary charging step may also be performed while pressurizing the secondary battery. Specifically, the primary charging step may be performed while the secondary battery is mounted on a jig formation device that can apply pressure to the secondary battery even during charging.

[0055] At this time, the pressure applied to the secondary battery is 0.1 kgf / cm 2 ~10kgf / cm 2 and preferably 0.3 kgf / cm 2 ~7.5kgf / cm 2 and more preferably 0.5 kgf / cm 2 ~5kgf / cm 2 It could be.

[0056] The method for manufacturing a lithium secondary battery according to the present invention may further include (d) aging the battery after (c) activation charging step, in which the secondary battery is aged under various conditions to accelerate the stabilization of the SEI film formed during the activation charging step.

[0057] The aging step (d) may be a room temperature aging process in which the secondary battery is aged for a predetermined time under room temperature / normal pressure conditions, or high temperature aging may be performed instead of room temperature aging depending on the purpose, or both room temperature aging and high temperature aging may be performed. The high temperature aging is performed by aging the battery in a high temperature environment, which may accelerate the stabilization of the SEI film, and the high temperature aging and room temperature aging processes may be performed sequentially on an initially charged battery.

[0058] In one specific example, the high-temperature aging can be carried out at a temperature of 50°C to 100°C, preferably 50°C to 80°C. The high-temperature aging can be carried out for 1 to 30 hours, preferably 2 to 24 hours. The high-temperature aging accelerates the stabilization of the SEI film, reduces the amount of voltage drop due to self-discharge in normal batteries, and further improves the ability to distinguish between good and low-voltage defective batteries.

[0059] In one specific example, the room temperature aging can be carried out at a temperature of 18° C. to 28° C., specifically 19° C. to 27° C., more specifically 20° C. to 26° C., and even more specifically 21° C. to 25° C. The room temperature aging can be carried out for 12 hours to 120 hours, or 18 hours to 72 hours.

[0060] In addition, the method for manufacturing a lithium secondary battery according to the present invention may further include a degassing process for discharging gas trapped inside the secondary battery to the outside. This degassing process may employ various degassing techniques known at the time of filing of the present invention. Since such degassing techniques are well known to those skilled in the art, further detailed description thereof will be omitted.

[0061] In addition, the method for manufacturing a lithium secondary battery of the present invention may further include a full discharge and full charge process in which the secondary battery is fully discharged to near SOC 0 and then charged to 95% or more of the designed capacity (SOC 95%) of the discharged secondary battery. The full discharge and full charge process may be performed once or repeated two or more times.

[0062] In one embodiment, the method for manufacturing a secondary battery according to the present invention may further include an additional aging process after the full discharge and full charge processes. The additional aging process is a process for stabilizing the secondary battery and may be carried out for 1 to 21 days. The additional aging process may include a monitoring (OCV tracking) process that includes measuring the open circuit voltage (OCV) of the battery at regular time intervals in order to screen for low-voltage defective batteries in which the voltage drops beyond the self-discharge of the battery.

[0063] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples of the present invention can be modified into various different forms, and the scope of the present invention should not be construed as being limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0064] <Manufacturing example: manufacturing of lithium secondary batteries> LiNi as the positive electrode active material 0.8 Co 0.1 Mn 0.1 95 parts by weight of O2, Li6Co as a positive electrode additive 0.7 Zn 0.3 A positive electrode mixture layer slurry was prepared by weighing 0.9 parts by weight of O4, 1.6 parts by weight of PVDF as a binder, and 2.5 parts by weight of carbon black as a conductive material in an N-methylpyrrolidone (NMP) solvent. The mixture layer slurry was applied to an aluminum foil, dried, and then rolled to form a positive electrode having a positive electrode mixture layer (average thickness: 130 μm).

[0065] A negative electrode mixture layer slurry was prepared by mixing 85 parts by weight of natural graphite as a carbon-based active material, 5 parts by weight of SiO (silicon oxide) as a silicon-based active material, 6 parts by weight of carbon black as a conductive material, and 4 parts by weight of PVDF as a binder in an N-methylpyrrolidone solvent. The slurry was then applied to copper foil to prepare a negative electrode having a negative electrode mixture layer (average thickness: 180 μm).

[0066] The resulting positive and negative electrodes were wound with a separator (thickness: approximately 16 μm) made of porous polyethylene (PE) film between them to produce a jelly-roll electrode assembly. The jelly-roll electrode assembly was then placed inside a cylindrical battery case, and an electrolyte was injected into the battery case to complete the battery assembly. The electrolyte used was 1M LiPF6 dissolved in an organic solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:7 (volume ratio) mixture.

[0067] Example 1 Ten lithium secondary batteries assembled according to the above manufacturing example were prepared, and sound waves were applied to the side of the battery while rotating the battery 360 degrees using a directional speaker. The frequency of the applied sound waves was 20 to 1000 Hz, and the sound waves were applied by sweeping the frequency, with the sound wave intensity being 110 dB. The sound wave application time was 5 minutes per session, and a total of two sessions were performed with a 1-minute time interval between each session.

[0068] The battery was then left to age for 24 hours at a temperature of 23°C.

[0069] <Example 2> Ten lithium secondary batteries assembled according to the above manufacturing example were prepared, and sound waves were applied to the side of the battery while rotating the battery 360 degrees using a directional speaker. The frequency of the applied sound waves was 20 to 1000 Hz, and the sound waves were applied by sweeping the frequency, with the sound wave intensity being 110 dB. The sound wave application time was 5 minutes per session, and a total of two sessions were performed with a 1-minute time interval between each session.

[0070] The battery was then left to age for 24 hours at a temperature of 23°C.

[0071] <Comparative Example> Ten lithium secondary batteries assembled according to the above manufacturing example were prepared and left to age at a temperature of 23° C. for 24 hours and 5 minutes.

[0072] <Experimental example: Confirmation of impregnation> The batteries of the above Examples and Comparative Examples were disassembled, and the area of the region impregnated with the electrolyte in the separator was measured, and the average values are shown in Table 1. The average impregnated area of each of Example 2 and Comparative Example 1 was expressed as a numerical value based on the average area measured in Example 1.

[0073] [Table 1]

[0074] As described above, it can be seen that the lithium secondary batteries manufactured according to the embodiments of the present invention have improved electrolyte impregnation properties when aged for the same period of time, including the process of removing bubbles using a speaker, compared to the lithium secondary batteries according to the comparative examples.

[0075] Therefore, it is expected that the method for manufacturing a lithium secondary battery of the present invention can shorten the time required for the pre-aging step for impregnation of the electrolyte.

Claims

1. (a) a battery cell assembly step of housing the electrode assembly in a battery case and injecting an electrolyte; (b) a pre-aging step of aging the battery cell into which the electrolyte has been injected; The (b) pre-aging step is (b-1) applying sound waves having a frequency of 20 Hz to 20,000 Hz to the assembled battery cell using a speaker; (b-2) Aging step of aging the battery cell the speaker is a directional speaker; In the step (b-1) of applying the sound wave, the carrier wave of the directional speaker is an ultrasonic wave.

2. 2. The method of claim 1, wherein in the step (b-1), the sound waves are applied locally to an area where bubbles are expected to be generated.

3. 2. The method of claim 1, wherein in the step (b-1), the sound wave is applied by sweeping the frequency of the sound wave.

4. 2. The method for manufacturing a lithium secondary battery according to claim 1, further comprising, after the (b) pre-aging step, (c) an activation charging step of charging the battery cell.

5. 2. The method of claim 1, wherein in the step (b-1), the sound waves are applied in a state where the battery cell is sealed.

6. 2. The method of claim 1, wherein in the step (b-1), the sound waves are applied in a state where the battery cell is not sealed.

7. 2. The method of claim 1, wherein in the step (b-1) of applying the sound wave, the frequency of the carrier wave of the directional speaker is 20 kHz to 100 kHz.

Images